Change gear in anger (4)
I believe the answer is:
rage
'anger' is the definition.
(I know that rage is a type of anger)
'change gear' is the wordplay.
'change' is an anagram indicator.
'gear' with letters rearranged gives 'RAGE'.
'in' acts as a link.
(Other definitions for rage that I've seen before include "Uncontrolled anger" , "Fury, passion" , "Temper - fashion" , "Intense anger" , "Gear up in violent temper" .)